A high-pressure fuel pump to which the present invention is applicable, has a plunger slidably fits to a cylinder and a pressurizing chamber whose volumetric capacity can be variable by a reciprocation of the plunger. The plunger pressurizes a fuel led into the pressurizing chamber through an inlet valve device, and discharges the fuel through an outlet valve device.
As a high-pressure pump of this type, the following types are known. One type is a high-pressure pump wherein a pressurizing chamber is formed in a pump body and the head of a cylinder protrudes into the pressurizing chamber (for example, a high-pressure pump described in the International Publication WO 02/055881 pamphlet). Another type is a high-pressure pump wherein a pressurizing chamber is formed in a cylinder (for example, a high-pressure pump described in Japanese Unexamined Patent Publication No. 2001-295770, 2003-49743, or the like).
Such high-pressure fuel pumps trend toward a higher pressure and a larger capacity of a fuel. In such a trend, this sort of high-pressure fuel pump is reciprocated, for example, constantly at a high speed of about 100 hertz (Hz) (at present, such a high speed occurs only in a high speed rotation region wherein an engine rotates at 6,000 rpm). In this condition, a lubricant (liquid film) formed in a gap between a cylinder and a plunger, which is formed by a part of the pressurized fuel from the pressurizing chamber, may become to be prone to deficient due to the heat generated by the slide of the plunger on a sled face of the cylinder. It may become a cause of that the sliding face (outer surface) of the plunger and the sled face (inner surface) of the cylinder are seized up or jammed even by the generation of a trifling amount of stress acting in the radial direction.
As a method for solving a similar problem in a similar field, known is a method of: forming a hole in the center of a piston corresponding to a plunger from the tip thereof in the axial direction; further forming a plurality of holes in the radial direction through which the hole in the axial direction communicates with the outer surface of the piston; and leading a part of the pressurized fuel from the piston side to the gap between the piston and a cylinder through the communicating holes (Japanese Unexamined Patent Publication No. H11(1999)-22493).
In the case of such a conventional configuration, in the state of compression in the process where the piston protrudes into the pressurizing chamber, a fuel in the pressurizing chamber is pressurized and supplied to the gap between the piston and the cylinder through the communicating pass (namely the hole formed in the plunger). However, positions of openings of the communicating pass on the slide face (on the outer surface) of the piston side always move in the axial direction in response to reciprocation of the piston, hence unstable force is loaded on the slide face in the radial direction, and jamming may rather increase.
Meanwhile, in a suction stroke state where the piston increases the capacity of the pressurizing chamber, the pressure in the pressurizing chamber lowers and hence the fuel at the gap between the piston and the cylinder may be extracted on the side of the pressurizing chamber through the communicating pass. On this occasion, the extraction state may continue while the positions of the openings of the communicating pass on the outer surface side of the piston move in the axial direction in response to the movement of the piston, hence the fuel at the gap between the piston and the cylinder is likely to be extracted. Thereby, lubricity may not improve less than expected, although such a complex communicating pass is provided.
Further, a diameter of the plunger in a high-pressure fuel pump to which the present invention is applied is as small as 10 millimeters (mm), thereby the strength of the plunger itself lowers if holes are formed in the plunger by a known technology, buckling tends to occur by the stress in the radial direction, and the original functions of the plunger may not be exercised.